Process for the batch filtering of suspensions

Abstract

For the batch filtration of suspensions by application of external pressure, after the pressing of the suspension in the press-chamber space and before entrance of the resultant press cake into at least one subsequent press-chamber space the structure of the press cake is changed by shearing forces and the press cake is curved and at least partially torn apart. In the case of municipal sludge the flaked sludge is subjected in a first pressing zone to slow increase of pressure and a pressure of up to about 1 bar and then is subjected in at least one following press zone to a pressure increase which is more rapid than in the first press zone, up to a pressure of about 3 bars.

What is claimed is: 
     
       1. A method of discontinuously filtering suspensions, comprising advancing a suspension stepwise along a path of travel through discrete separated first and second pressure chambers, applying external pressure to said suspension to effect filtering of the suspension by compression in said first pressure chamber to form a filter cake, the suspension being stationary in said first pressure chamber during said compression of said suspension, applying shearing forces to the surface of the resulting filter cake to alter its structure in the travel of said filter cake from the first filter chamber to the second filter chamber and applying external pressure to the filter cake to effect filtering of the cake by compression in said second pressure chamber, said filter cake being stationary in said second pressure chamber during said compression of said cake. 
     
     
       2. A method as claimed in claim 1 wherein said suspension is advanced along said path of travel by being transported therealong between spaced filter members. 
     
     
       3. A method as claimed in claim 2 wherein each of said pressure chambers is formed by clamping said filter members together at spaced locations. 
     
     
       4. A method as claimed in claim 2 wherein said shearing forces are produced by advancing said filter members at different speeds between the first and second pressure chambers. 
     
     
       5. A method as claimed in claim 1 comprising bending said filter cake and effecting at least partial tearing thereof during travel of said cake from said first to said second pressure chamber. 
     
     
       6. A method as claimed in claim 5 wherein the filter cake is advanced from the second pressure chamber to a third pressure chamber, said filter cake being bent during travel from the second to the third pressure chamber in a direction different from that in going from the first to the second pressure chamber. 
     
     
       7. A method as claimed in claim 6 wherein the filter cake is bent in opposite directions. 
     
     
       8. A method as claimed in claim 1 wherein the pressure applied to the filter cake is greater in said second chamber than in said first chamber. 
     
     
       9. A method as claimed in claim 1 wherein the suspension is floculated sludge, the floculated sludge being filtered in the first pressure chamber by a pressure increasing to about 1 bar, and in the second pressure chamber by a pressure increasing, at a faster rate than the pressure in said first chamber, to about 3 bars. 
     
     
       10. A method as claimed in claim 9 wherein said sludge is filtered again in a third pressure chamber by the application of a pressure increasing, at a faster rate than the pressure in said second chamber, to about 10 bars. 
     
     
       11. A method as claimed in claim 1 wherein each pressure chamber is formed by opposed plates located at fixed stations along said path of travel and between which the filter members and suspension are advanced, said external pressure being applied to the suspension by relatively moving said plates towards one another. 
     
     
       12. A method as claimed in claim 11 wherein said plates are parallel to one another and are relatively moved perpendicularly to one another. 
     
     
       13. A method as claimed in claim 1 wherein said path of travel is U-shaped. 
     
     
       14. A method as claimed in claim 2 wherein said filter members are endless bands which are transported in parallel relationship along said path of travel.